Open Access Embargo
Date of Award
Chronic stress disrupts insulin signaling, predisposing human populations to diabetes, cardiovascular disease, Alzheimer’s Disease, and other metabolic and neurological disorders, including post-traumatic disorders (PTSD). Thus, efficient recovery from stress optimizes survival. However, stress recovery in humans is difficult to study, but is much easier to dissect in model organisms. The worm genetic model Caenorhabditis elegans can switch between stressed and non-stressed states, and this switch is largely regulated by insulin signaling. Previously, the Alcedo lab proposed that insulin-like peptides (ILPs), which exist as multiple members of a protein family in both C. elegans and humans, implements a combinatorial coding strategy to control the switch between the two physiological states. The concept of combinatorial coding has led to the identification of an inter-ILP network, where one ILP, ins-6, is a major node of the network. This is consistent with ins-6 as the most pleotropic of all ILPs that have been tested. ins-6 has also been shown to be the most important ILP in promoting stress recovery in C. elegans. Because of its central role in the ILP network and in stress recovery, for my thesis I identified mechanisms through which INS-6 regulates the network and an animal’s recovery from stress.
Under optimal environments, ins-6 mRNA is endogenously expressed in the cell bodies of one or two chemosensory neurons, ASI and ASJ, in the developing animal. However, upon stress-induced developmental arrest, known as dauer, ins-6 mRNA is only limited to the ASJ sensory neurons. I discovered that ins-6 mRNA from ASJ is also surprisingly transported to the axonal nerve ring bundle of stressed animals, but lost from the nerve ring after recovery from stress. Consistent with the existence of an inter-ILP network, insulin signaling regulates ins-6 mRNA transport, which also requires the activities of specific kinesins. This transport additionally depends on the untranslated regions of ins-6 mRNA, but these regions are insufficient for transport. More importantly and in collaboration with other members of the Alcedo lab, we showed that axonal ins-6 mRNA facilitates stress recovery, where high axonal ins-6 mRNA promotes faster recovery and low axonal ins-6 mRNA delays recovery. Moreover, I demonstrated the existence of axonal Golgi bodies, whose mobilization are enhanced during stress. Together my data suggest that stress stimulates the axonal transport of ILP mRNAs, which are then locally translated and packaged for secretion--a mechanism that promotes plasticity during stress and optimal stress recovery.
To identify additional regulators of ins-6 mRNA, I also performed, together with other members of the lab, a forward genetic screen for mutants that alter ins-6 transcription during stress. Through whole-genome sequencing, one of the five mutants we isolated is potentially a mutation in an innexin gap junction protein. Since innexins have been shown to regulate neural activity, I tested the hypothesis that neural activity will also affect axonal ins-6 mRNA transport. Interestingly, I found that a synaptic transmission mutant, which should have low neural activity, increases axonal ins-6 mRNA and alters neurite morphology.
My thesis study raises an intriguing hypothesis: stress modulates neurite activity and morphology, which in turn promote ILP mRNA transport to the axons. The axonal localization of an ILP mRNA also uncovers a novel mechanism of insulin signaling during stress. Because of the high degree of conservation between C. elegans and humans and the effects of altered insulin signaling in stressed brains, my findings should advance our understanding of how a nervous system recovers from stress. The work described in this thesis should lead to potential therapies for stress management to promote better health.
Chandra, Rashmi, "Stress-Dependent Regulation Of A Major Node Of The Insulin-Like Peptide Network That Modulates Survival" (2019). Wayne State University Dissertations. 2236.
Available for download on Tuesday, August 31, 2021